Brood Parasitic Birds: The success of the Shiny Cowbird without mimicry

Obligate brood parasitic birds outsource the time intensive job of maternal care by laying their eggs in the nest of other species1. Sometimes the host is tricked via mimicry into rearing the parasitic chick, and other times a different approach is utilised by the parasitic bird. The disuse of mimicry in the parasitic Shiny Cowbird (Molothrus bonariensis) was discussed by Dr Ros Gloag (from University of Sydney) in her seminar at University of Sydney.

Typically, a parasitic bird can use mimicry as a means to evade host defences1. For example, a cuckoo can mimic a bird of prey so that the hosts will flee the nest, giving the cuckoo free range to lay its eggs1. Cuckoos can also mimic host eggs to avoid removal by the host species1. Furthermore, cuckoos can ‘tune in’ to the hosts communication, with cuckoo chicks replicating the sound of the host chicks1. However, as mentioned by Dr Gloag, not all brood parasites rely on mimicry but they still succeed. This has been seen in the Shiny Cowbird in South & Central America – a generalist brood parasitic bird that doesn’t mimic eggs, and doesn’t provide visual or vocal mimicry1.

 Juvenille cowbird being fed by a sparrow A Shiny Cowbird juvenile being fed by an adult Rufous-collared Sparrow (Zonotrichia capensis). Retrieved from http://en.wikipedia.org/wiki/Shiny_Cowbird

No vocal mimicry in chicks

The House Wren (Troglodytes aedon) is the most common host of the Shiny Cowbird1, and according to Dr Gloag, they have very different calls. It would hence be expected that the House Wren would realise that the Shiny Cowbird is not its own, and reject it from the nest. But quite the opposite occurs. The study by Gloag and Kacelnik (2013) found that the House Wren attended to nests with Shiny Cowbird brood calls 20% more than nests with House Wren brood calls (see Figure 1).

Why would a House Wren attend to a Shiny Cowbirds brood call more than its own chicks? There are multiple hypotheses for this. It is possible that the House Wren is only responding to the extent of sound from the nest, and that call structure does not matter2 (quantity not quality). Furthermore, it is hypothesised by Gloag and Kacelnik (2013) that the Shiny Cowbird call is simulating the call of older House Wren chicks, although this remains to be tested2.

attendance rates Figure 1: Mean attendance rates of the House Wren to nests during the broadcast of fledgling calls. Altered from Gloag and Kacelnik (2013, p. 106)

No vocal mimicry in adults

If the Shiny Cowbirds don’t provide any vocal mimicry, how do they get the host away from the nest to lay? Typically they don’t. In the example of the Chalk-browed Mockingbirds (Mimus saturninus), most Shiny Cowbirds endure a violent mobbing by the Chalk-browed Mockingbirds, whilst at the same time trying to lay their eggs1. In addition, the Shiny Cowbirds will also attempt to destroy any eggs that are already in the clutch – still whilst experiencing the mobbing5. An epic task!

As shown by Gloag et al. (2013), the mobbing actually doesn’t really deter egg laying by the Shiny Cowbirds. However, it does reduce the number of eggs that are broken by the Shiny Cowbirds5. This means that by mobbing the Shiny Cowbirds, the Chalk-browed Mockingbirds will have a higher chance that their eggs will be successfully reared5.

No egg mimicry

As stated before, Shiny Cowbirds don’t mimic the eggs of their host species. So even if a Cowbird lays its egg in the host nest, why wouldn’t it just reject it? This may be explained by the Clutch Dilution Hypothesis as proposed by Sato, Mikamf & Ueda (2010) (see Figure 2) – an evolutionary trade off1.

In a multiple-parasite system there are multiple cases of parasite invasion of the nest, leading to several events of eggs removed or ruined within the clutch3. By having multiple parasite eggs in the nest, there is a lesser chance that the host egg will be targeted in a future attack3. However, if the host removes all parasite eggs, than the host eggs are more vulnerable to future attacks1.

  Clutch dilutionFigure 2: Clutch dilution hypothesis. H refers to host eggs; P refers to parasite eggs. Source: S. Cardenzana

The study of brood parasitism in the Shiny Cowbird has evidently increased understanding of co-evolution between the Shiny Cowbird and its various host species. When experiencing high cases of parasitism, the host species has to therefore develop a trade-off between their own eggs surviving and the rearing of parasitic birds3. Overall this provides an interesting example of where mimicry is not used to parasitise host species, unlike what is typically seen in various cuckoo species.

References

  1. Gloag, R. (2014, April 4). Trickery without mimicry in brood parasitic birds. School of Biological Science Seminar Series. Conducted from University of Sydney, Camperdown, NSW.
  1. Gloag, R., & Kacelnik, A. (2013). Host manipulation via begging call structure in the brood-parasitic shiny cowbird. Animal Behaviour, 86(1), 101-109. doi: 10.1016/j.anbehav.2013.04.018
  1. Gloag, R., Fiorini, V. D., Reboreda, J. C., & Kacelnik, A. (2012). Brood parasite eggs enhance egg survivorship in a multiply parasitized host. Proceedings of the Royal Society B: Biological Sciences, 279(1734), 1831-1839. doi: 10.1098/rspb.2011.2047
  1. Gloag, R., Fiorini, V. D., Reboreda, J. C., & Kacelnik, A. (2013). The wages of violence: mobbing by mockingbirds as a frontline defence against brood-parasitic cowbirds. Animal Behaviour, 86(5), 1023-1029. doi: 10.1016/j.anbehav.2013.09.007
  1. Sato, N. J., Mikamf, O. K., & Ueda, K. (2010). The egg dilution effect hypothesis: a condition under which parasitic nestling ejection behaviour will evolve. Ornithological Science, 9(2), 115-121. doi: 10.2326/osj.9.115
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